Disclosed is a liquid knife (200), comprising a water inlet opening (212), a water outlet groove (214), and a waste storage tank (225), wherein the water outlet groove (214) is in communication with an upper part of the waste storage tank (225), and liquid enters via the water inlet opening (212), enters the waste storage tank (225), and is then discharged from the water outlet groove (214). The liquid knife collects residue by means of the waste storage tank (225), and this can effectively prevent the water outlet groove of the liquid knife from becoming blocked.